The present invention relates to a semiconductor strain sensor such as a semiconductor accelerometer and a pressure sensor, and methods of manufacturing such semiconductor strain sensing devices.
A conventional semiconductor accelerometer is provided with a beam portion on its sensor chip (silicon chip) which has piezo-resistors so as to form a bridge circuit. The beam portion, being disposed in the air, has a resonance frequency determined by its shape and the resonance output thereof, which is an output value obtained when a cantilever, the beam portion, enters into a resonance state, reaches about 1000 times the output under the normal conditions, thereby providing the possibility that the beam portion is easily broken. For eliminating this problem, the sensor chip is adhered or bonded to a stem, and a shell is provided to surround a sensor chip by means of the projection welding technique before enclosing therein damping liquid such as silicon oil.
There is a problem which arises with this arrangement, however, in that the stem welding strain is propagated up to the sensor chip so that the offset voltage of the bridge circuit formed on the sensor chip varies greatly. This will hereinbelow described in detail with reference to FIGS. 1 to 4. In FIGS. 1 and 2 showing a press process for manufacturing a stem illustrated as numeral 9, a belt-like steel plate 1 is initially machined so as to be partially removed to form at least one stem formation portion 2. Illustrated as numeral 3 is the removed portion. Secondly, coining is performed by means of a press so as to thin the circumferential portion 4 of the stem formation portion 2 to establish a step. Further, the stem formation portion 2 is machined by a punch so as to form six lead holes 6 each extending in directions perpendicular to surfaces of the belt-like steel plate 1. At this time, as shown in FIG. 2 which is a cross-sectional view taken along a line 2--2 in FIG. 1, the stem 9 partially warps due to the lead-hole formation, hence followed by the trimming process. Thereafter, a shell (10) is welded on the stem 9 by means of the projection welding with silicon oil being enclosed therein. Here, the stem 9 and the shell 10, as shown in FIGS. 3 and 4, are put on top of each other and disposed between upper and lower electrodes 7 and 8. The stem 9 is arranged to come into contact with the shell 10 at a portion indicated by a two-dot chain line (a) in FIG. 4 and fixedly welded thereto through pressurization. At this time, since a great force (about one ton) is applied to the stem 9, the stem 9 warps so as to produce a strain (about 190 micro (.lambda.) strain). This strain is propagated into a sensor chip illustrated as numeral 11, causing the sensor characteristic to vary.